1. Field of the Invention
The present invention relates to apparatus for establishing a relatively uniform charge level on charge-retaining materials, in general, and to such apparatus for establishing a uniform charge level on a moving web of such materials, in particular.
2. Description of the Prior Art
The presence of electrostatic charges on charge-retaining materials causes problems in many industries. In the photographic industry, for example, electrostatic charges on potential photographs or film units within a light-tight film cassette containing a plurality of film units for use in an "instant" type photographic camera, such as that sold by Polaroid Corporation, Cambridge, Mass., under its registered trademark SX-70, will often cling to one another with such intensity as a result of the force of attraction developed by such electrostatic charges, that proper ejection of an exposed film unit from said film cassette can be prevented if the effects of such charges are not controlled. In the SX-70 photographic film units mentioned above, for example, electrostatic charges are controlled by controlling the charge levels on components of said film prior to final film unit assembly.
In my copending U.S. patent application Ser. No. 183,326, filed Sept. 2, 1980, a brush-like device is employed to establish a desired electrostatic charge level on a moving web of charge-retaining material by passing said web through a relatively intense electrostatic field generated by said device when it is electrically connected to a relatively low potential DC source of suitable magnitude and polarity. A similar but more limited disclosure of said brush-like device is contained at page 70 in the February 1980 issue of Research Disclosure.
A shortcoming associated with electrostatic charge controlling conductive bristle brushes employed to establish an electrostatic charge on charge-retaining materials is their inability to continuously place a uniform charge on such materials for extended periods of time. When controlling electrostatic charges on, for example, a moving web of charge-retaining material with a conductive bristle brush of the type described above, streaks or strip-like portions of said web will fail to reach the same charge level as other web portions after several days of continuous web charging with such a conductive bristle brush.